Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali Electrolytes: Ceria Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova
Bibliography 1. N.Q. Minh, T. Takahashi: Science and technology of ceramic fuel cells Elsevier 1995 2. M. Mogensen et al. Solid State Ionics 129 (2000) 63-94 3. H. Inaba et al. Solid State Ionics 83 (1996) 1-16 4. V.V. Kharton et al. J. Mat. Science 36 (2001) 1105-1117
Electrical conductivity of fluoride-type oxides
Pure CeO 2 = pale yellow The ceria structure is known to tolerate a considerable reduction without phase change, especially at elevated temperatures. Such ceria is blue and turns almost black when grossly nonstoichiometric. The colour of CeO 2 is also sensitive to the presence of other lanthanides. 0.02% of Pr results in a brownish-yellow colour
Ceria Pure CeO 2 = cubic fluorite structure up to its melting point (does not need any stabilization) Depending on T and P O2 ceria exhibits a large oxygen deficiency with the formula CeO 2-δ (δ as large as 0.3). For small oxygen deficiencies (δ < 0.001) doubly ionized oxygen vacancies are the principal ionic defects, for large oxygen deficiencies a transition toward singly ionized vacancies has been observed pure ceria = n-type electrical conduction; ionic conductivity is negligible Ce 4+ ion radius is sufficiently large: a variety of dopant can be incorporated Ceria doped with divalent or trivalent oxides shows relatively high oxygen-ion conductivity at elevated temperatures: compared to YSZ shows a higher conductivity and a lower conduction activation energy
Ceria reduction Ceria undergoes reduction (to Ce III) at low P O2 : this restricts the range of O 2 partial pressure CeO 2-x The process of ceria reduction Oxide vacancies may also be introduced by doping with oxides of metals with lower valencies, e.g., by dissolution of CaO or Gd 2 O 3 : Oxide vacancies may be removed by doping with oxides of higher valencies: Law of mass action [Ce Ce ] = 2x [V O ] = x Undoped ceria [Ce Ce ] = 2x [V O ] = constant Doped ceria
Defects Association The theory of non-interactingdefects predicts an n-value of 6 The high n-values are not explainable in terms of non-interacting defects Discrete ordering phases separated by two-phase regions G O2 = RT ln P O2 Relative partial free energies, G, at 1100 C of CeO as a function of composition (log x)
1950s Brauer and Gradinger phases CeO 1.812, CeO 1.782, and CeO 1.719, formed as a result of ordering in the cation and anion sublattices. Bevan = δ-phase, Ce 11 O 20 (rhombohedral symmetry, exists over a broad composition range) ceria can be reduced to nonstoichiometric compositions, CeO 2-x, where 1.7 2-x 2. Ce n O 2n-2 Four distinct phases corresponding to n = 7 (1.74), 9 (1.79), 10 (1.8) and 11 (1.818). Riess Phase Diagram.
n = 17.90 n = 14.90 n = 4 n = 5 Diagram of subregions with possible ordered intermediate phases in the α-phase for the cerium oxygen system 1.805 1.854 1.989-1.984 1.749
Molar Volume Changes in the molar volume of CeO 2-x vs. composition for the transformation α+δ α+α and α+δ α +δ (sx) on heating; (dx) on cooling Undercooling during temperature decrease
Kim Conductivity and dopant ion radius the nonstoichiometry may be regarded as a simple solid solution of Ce 2 O 3 in CeO 2. lattice parameter should follow Vegard s rule, i.e., a linear relationship exists between lattice parameter and the concentration of the solute. The slope of this straight line is termed Vegard s slope. a (nm) = lattice constant of ceria r (nm) is the difference in ionic radius of the kth dopant Ce 4+ -radius (eightfold coordination) is 0.097 nm according to Shannon z k is the valence (z k - 4) m k = mole percent of the k th dopant in the form of MO 2. Ce 2 O 3 Ln 2 O 3
Ceria reduction as a doping Kim Percent expansion of cerium dioxide versus nonstoichiometric composition at 900 C. (- - -) Theoretical slope; ( ) best fit slope. Expansion: Ce(III) ion radius is bigger than Ce(IV) Oxide vacancy radius = 0.1164 nm (RT)
Solubility of other oxides and change in lattice parameters Kim: the solubility limit of a solute depends on the elastic energy, W, which is introduced in the lattice due to differences in ionic radius. The larger the elastic energy per substituted ion, the lower is the solubility. a 0 (nm) = lattice constant of ceria a (nm) change in lattice parameter (governed by Vegard s slope) G shear modulus CRITICAL RADIUS = radius giving a Vegard s slope of zero (0.1106 to 0.101 nm)
Determination of the critical ionic radius r. The y- axis is the slopes of each individual lattice parameter as a function of dopant concentration, x.
Doped ceria Used dopants: La 2 O 3, Y 2 O 3, Sm 2 O 3, Gd 2 O 3, Gd 2 O 3 +Pr 2 O 3, CaO, SrO Arrhenius plots of ionic conductivities of doped ceria compounds
Conductivity and dopant ion radius The ionic conductivity increases with increasing ionic radius, from Yb to Sm, but decreased at r > 0.109 nm. (CeO 2 ) 0.8 (LnO 1.5 ) 0.2 as a function of radius of dopant ion. Ce(IV) = 128 pm Ce(III) = 111 pm Ionic conductivity of doped ceria
Doped ceria Sm 3+ among the rare earth oxide and Ca 2+ among alkaline earth = maximum electrical conductivity Ionic conductivity of doped ceria at 1073 K against the radius of dopant cation In the horizontal axis the critical radius of M 2+ and M 3+ The maximum electrical conductivity is due to the similar ionic radius as the host ion = minimum association entalpy between dopant ion and oxygen vacancy
Mg, Ba - Doped ceria The electrical conductivities when doping with MgO and BaO are exceptionally low = insufficient solubility of these oxides in ceria. Sm Sr Ca Ba Mg Lattice constants of ceria based oxides (CeO 2 ) 1-x (MO y ) x as a function of dopant concentration, x
Conductivity and dopant ion radius CeO 2 -Ln 2 O 3 1. Conductivities are higher for different concentrations depending on the dopant 2. The minimum E att depends on dopant cations Low T activation entalpy and electrical conductivity of Y- doped ceria as a function of dopant concentration There are some interactions between dopant ions and oxygen vacancies Activation energy against dopant concentration for various rare earthdoped cerias
Conductivity and dopant ion radius 1. Difetti liberi 2. Difetti carichi 3. Difetti neutri CeO 2-x x < 10-3 CeO 2-x 10-3 < x < 10-2 CeO 2-x 10-3 x > 10-2
Butler et al. and Catlow modelled the effect of the dopant ion radius on the dopant vacancy interaction in CeO 2, and thus on the ionic conductivity. Calculated binding energy of oxide vacancies: an oxygen vacancy with one, two, three and four nearest neighbour dopant ions for dopant ions with different ion size and charge.
Free Vacancies A 1, A 2, Associative m = Migration W = number of orientation of the associate C M = total dopant concentration Uncharged Associated Defects Dopant cation radius Low Order Double Doping Charged Associated Defects
Conductivity and dopant ion radius MO 2 -Ln 2 O 3
Grain Boundary Conductivity Strong influence of the preparation procedure Some grain boundary resistance is most often seen even in high purity material, especially at low temperature Plot of σt versus 1/T for the CeO :0.3% CaO sample 1. amorphous glassy phase in the grain boundaries caused by impurities. 2. microporosity in the boundaries 3. segregation of the dopant ions. Bulk conductivity Bulk conductivity + Grain Boundary
Grain Boundary Conductivity Doping reduces the effect of grain boundary (dopant segregation) Bulk Total Total Total Bulk Plot of σt versus 1/T for the CeO 2 : 0.3% CaO sample Segregation of high-resistivity phases Doping extraction from bulk toward surface: grains become practically undoped (micron grains) Plot of σt versus 1/T for the CeO 2 : 15% CaO sample
GBC and sintering temperature Complex impedance spectra of CGO20 ceramics sintered in air at 1873 (open simbols) and 1773 K for 2 hours (full simbols). SEM micrographs of CGO20 ceramics sintered in air at 1873 (A) and 1773 K (B) for 2 hours.
T dependence of the total conductivity in air: (A) CGO20 and CGO20- based ceramics, (B) CGO10 and CGO10-based ceramics. Small additions of several cations seem to be ineffective, probably due to segregation of dopants at the grain boundaries
Doped ceria electrical conductivity increases at low oxygen partial pressures Electrolytic Domain? Conductivity of doped ceria as a function of oxygen partial pressure
Doped ceria reduction The reduction (i.e. the electronic conductivity) of doped ceria under reducing atmospheres can be minimized by modifying the dopant: Another possibility is to suppress the reduction of ceria by coating the solid with a film of a more stable ionic conducting compound (CeO 2 ) 0.8 (Sm 2 O 3 ) 0.2 coated with a YSZ thin film (2 µm) on the fuel side produced a stable SOFC electrolyte Electrolytic and ionic domain boundaries of doped ceria Conductivities of (ZrO 2 ) 0.9 (Y 2 O 3 ) 0.10 as a function of oxygen partial pressure
Doped ceria reduction The replacement of 3%mol Gd by Pr in Ce 0.8 Gd 0.2 O 2-δ improves the electrolytic domain of the material by nearly two orders of magnitude without significantly affecting ionic conductivity. Temperature dependence of the electrolytic domain boundary of CeO 2 -based electrolytes at low oxygen pressures.
The chemical compatibility?
At temperatures <1000 C (operating T) = little interfacial phase formation. At higher temperatures (fabrication)... Ce 1-x Gd x O 2-y with 8YSZ Reaction after heat treatment at 1300 C for 72 h: formation of a cubic-like phase Ni/YSZ anode + Ce(Gd)O 2-y electrolyte sintered at 1300 C = formation of Gd 2 Zr 2 O 7 and Gd 2 NiO 4 : high resistivity & poor cell performance Chromium based interconnects/gd doped ceria: No reaction between (La,Sr)CrO 3 and Ce 0.8 Gd 0.3 O 1.9 at 1600 C. SrCrO 4 reacted with CGO via a Sr Cr O liquid phase forming an unknown phase at the interface and grain boundaries, consisting of Ce, Sr, Gd and Cr. CaCrO 4 reacted with CGO via a liquid Ca Cr O phase, transforming CGO grains into an unknown phase. Cr 2 O 3 compatible with CGO for temperatures below 1400 C in air.
In general, fracture of ceramics is caused by flaws originating from the fabrication, and thus, the fabrication method is of most importance. YSZ Bonding Strength = 300-400 MPa YSZ Fracture Toughnes = 3MPa m 1/2